Charged particle beam devices like electron microscopes, focussed ion beam devices, or electron beam pattern generators are required to deliver an ever increasing spatial resolution for inspecting or structuring specimens like semiconductor wafers, masks, biological specimens, and the like. A high spatial resolution can only be achieved if the focus spot size of the charged particle beam is made sufficiently small. Focussing a charged particle beam to a small spot size, however, requires a tight control of the focussing electric and/or magnetic fields.
Unfortunately, in practice, any conducting component nearby the charged particle beam may be a source for distorting a focussing electric field. Therefore, whenever a component nearby the charged particle beam is moved with respect to the beam during operation, the focussing quality of the charged particle beam source may suffer.
Focussing electric field distortions also occur when the specimen itself is moved. This situation arises when, e.g., the charged particle beam device is used to inspect or structure a specimen at different landing angles. The landing angle refers to the angle between the inspected or structured surface of the specimen and the direction of the incoming (primary) charged particle beam. Inspecting a specimen at different landing angles may significantly increase information on the surface of the specimen, like surface topology, chemical surface structure, etc.
Usually, the landing angle is adjusted by means of some tilting mechanism which tilts the charged particle beam device with respect to the surface of the specimen. FIGS. 1a and 1b illustrate an example where a semiconductor wafer 3 is inspected by a scanning electron microscope 1 (SEM) at two different landing angles 42. In FIG. 1a, the specimen 3 is inspected at a first landing angle 42 of 90 degrees, while in FIG. 1b, the specimen is inspected at a second landing angle 42 of 45 degrees. Note that, while in FIG. 1a-b the SEM 1 becomes tilted in order to obtain a tilted landing angle, other types of SEMs use a set-up where the specimen becomes tilted in order to obtain a tilted landing angle.
The SEM 1 of FIGS. 1a and 1b is comprised of a beam tube 20 having an electron beam source 5, e.g. a thermal field emission cathode, to generate an electron beam 7, a high voltage beam tube 9 to accelerate the electron beam 7 up to an energy controlled by an anode voltage Vanode, a condenser 11 to improve the electron beam shape, a magnetic focussing lens 13 and an electrostatic focussing lens 14 to focus the electron beam 7 onto the wafer 3. The SEM 1 of FIG. 1a and 1b further comprises an in-lens detector 15 to detect and evaluate the signal of the secondary charged particles 17 which are generated by the primary electron beam 7 on the wafer 3.
The magnetic focussing lens 13 of FIG. 1a and 1b consists of a coil 24 and a yoke 26 shaped to generate a focussing magnetic field for the primary electron beam 7. The electrostatic focussing lens 14 of FIG. 1a and 1b is comprised of the lower-end elements 9a of the high voltage beam tube 9, the cone-like shaped elements 26a (“conical cap”) of yoke 26, and apertures 106 at the apices of the respective elements. The focussing electric field is defined by the geometry of the lower-end element 9a, of the conical cap, their apertures 106 and by the voltages V1 and V2 between the wafer 3 and, respectively, the conical cap 26a and the high voltage beam tube 9 (for simplicity of the drawings, the voltages V1, V2 and Vanode are only shown in FIG. 1a). As it turns out, if the electric field between the conical cap 26a and wafer 3 is adjusted in such a way that it decelerates the primary electron beam 7, the spatial resolution of the probing primary electron beam can be increased when combined with a magnetic focusing field. More details about the combined electrostatic and magnetic focussing lens, and about the SEM of FIG. 1a in general, can be found in “High Precision electron optical system for absolute and CD-measurements on large specimens” by J. Frosien, S. Lanio, H. P. Feuerbaum, Nuclear Instruments and Methods in Physics Research A, 363 (1995), pp. 25-30.
In FIG. 1b, the beam tube 20 is tilted by 45 degrees with respect to the wafer 3 to inspect the wafer 3 at a second landing angle 42 of 45 degrees. In the case of FIGS. 1a and 1b, a controlled tilting has been realized by a tilting mechanism 22 which enables the SEM to inspect any location on the wafer at (at least) two different landing angles 42. Further, due to the cone-like shaped elements 26a of the yoke 26, it is possible to tilt the SEM while maintaining a short working distance between the focussing lens 14 and the specimen 3, as can be seen from FIG. 1b. The cone-like shape of the cone-like shaped elements 26a of the yoke 26 prevents the beam tube 20 from touching or scratching on the specimen 3 when tilted, without having to give up on the short working distance.
The charged particle beam device of FIGS. 1a and 1b allows for an inspection of a specimen at different predetermined landing angles. However, as it turns out, changing the landing angle away from a perpendicular direction can severely reduce the spatial resolution of a charged particle beam device.